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  confidently conclude that if a thing belongs to the class of unmoved

  movents that are also themselves moved accidentally, it is

  impossible that it should cause continuous motion. So the necessity

  that there should be motion continuously requires that there should be

  a first movent that is unmoved even accidentally, if, as we have said,

  there is to be in the world of things an unceasing and undying motion,

  and the world is to remain permanently self-contained and within the

  same limits: for if the first principle is permanent, the universe

  must also be permanent, since it is continuous with the first

  principle. (We must distinguish, however, between accidental motion of

  a thing by itself and such motion by something else, the former

  being confined to perishable things, whereas the latter belongs also

  to certain first principles of heavenly bodies, of all those, that

  is to say, that experience more than one locomotion.)

  And further, if there is always something of this nature, a movent

  that is itself unmoved and eternal, then that which is first moved

  by it must be eternal. Indeed this is clear also from the

  consideration that there would otherwise be no becoming and

  perishing and no change of any kind in other things, which require

  something that is in motion to move them: for the motion imparted by

  the unmoved will always be imparted in the same way and be one and the

  same, since the unmoved does not itself change in relation to that

  which is moved by it. But that which is moved by something that,

  though it is in motion, is moved directly by the unmoved stands in

  varying relations to the things that it moves, so that the motion that

  it causes will not be always the same: by reason of the fact that it

  occupies contrary positions or assumes contrary forms at different

  times it will produce contrary motions in each several thing that it

  moves and will cause it to be at one time at rest and at another

  time in motion.

  The foregoing argument, then, has served to clear up the point about

  which we raised a difficulty at the outset-why is it that instead of

  all things being either in motion or at rest, or some things being

  always in motion and the remainder always at rest, there are things

  that are sometimes in motion and sometimes not? The cause of this is

  now plain: it is because, while some things are moved by an eternal

  unmoved movent and are therefore always in motion, other things are

  moved by a movent that is in motion and changing, so that they too

  must change. But the unmoved movent, as has been said, since it

  remains permanently simple and unvarying and in the same state, will

  cause motion that is one and simple.

  7

  This matter will be made clearer, however, if we start afresh from

  another point. We must consider whether it is or is not possible

  that there should be a continuous motion, and, if it is possible,

  which this motion is, and which is the primary motion: for it is plain

  that if there must always be motion, and a particular motion is

  primary and continuous, then it is this motion that is imparted by the

  first movent, and so it is necessarily one and the same and continuous

  and primary.

  Now of the three kinds of motion that there are-motion in respect of

  magnitude, motion in respect of affection, and motion in respect of

  place-it is this last, which we call locomotion, that must be primary.

  This may be shown as follows. It is impossible that there should be

  increase without the previous occurrence of alteration: for that which

  is increased, although in a sense it is increased by what is like

  itself, is in a sense increased by what is unlike itself: thus it is

  said that contrary is nourishment to contrary: but growth is

  effected only by things becoming like to like. There must be

  alteration, then, in that there is this change from contrary to

  contrary. But the fact that a thing is altered requires that there

  should be something that alters it, something e.g. that makes the

  potentially hot into the actually hot: so it is plain that the

  movent does not maintain a uniform relation to it but is at one time

  nearer to and at another farther from that which is altered: and we

  cannot have this without locomotion. If, therefore, there must

  always be motion, there must also always be locomotion as the

  primary motion, and, if there is a primary as distinguished from a

  secondary form of locomotion, it must be the primary form. Again,

  all affections have their origin in condensation and rarefaction: thus

  heavy and light, soft and hard, hot and cold, are considered to be

  forms of density and rarity. But condensation and rarefaction are

  nothing more than combination and separation, processes in

  accordance with which substances are said to become and perish: and in

  being combined and separated things must change in respect of place.

  And further, when a thing is increased or decreased its magnitude

  changes in respect of place.

  Again, there is another point of view from which it will be

  clearly seen that locomotion is primary. As in the case of other

  things so too in the case of motion the word 'primary' may be used

  in several senses. A thing is said to be prior to other things when,

  if it does not exist, the others will not exist, whereas it can

  exist without the others: and there is also priority in time and

  priority in perfection of existence. Let us begin, then, with the

  first sense. Now there must be motion continuously, and there may be

  continuously either continuous motion or successive motion, the

  former, however, in a higher degree than the latter: moreover it is

  better that it should be continuous rather than successive motion, and

  we always assume the presence in nature of the better, if it be

  possible: since, then, continuous motion is possible (this will be

  proved later: for the present let us take it for granted), and no

  other motion can be continuous except locomotion, locomotion must be

  primary. For there is no necessity for the subject of locomotion to be

  the subject either of increase or of alteration, nor need it become or

  perish: on the other hand there cannot be any one of these processes

  without the existence of the continuous motion imparted by the first

  movent.

  Secondly, locomotion must be primary in time: for this is the only

  motion possible for things. It is true indeed that, in the case of any

  individual thing that has a becoming, locomotion must be the last of

  its motions: for after its becoming it first experiences alteration

  and increase, and locomotion is a motion that belongs to such things

  only when they are perfected. But there must previously be something

  else that is in process of locomotion to be the cause even of the

  becoming of things that become, without itself being in process of

  becoming, as e.g. the begotten is preceded by what begot it: otherwise

  becoming might be thought to be the primary motion on the ground

  that the thing must first become. But though this is so in the case of
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  any individual thing that becomes, nevertheless before anything

  becomes, something else must be in motion, not itself becoming but

  being, and before this there must again be something else. And since

  becoming cannot be primary-for, if it were, everything that is in

  motion would be perishable-it is plain that no one of the motions next

  in order can be prior to locomotion. By the motions next in order I

  mean increase and then alteration, decrease, and perishing. All

  these are posterior to becoming: consequently, if not even becoming is

  prior to locomotion, then no one of the other processes of change is

  so either.

  Thirdly, that which is in process of becoming appears universally as

  something imperfect and proceeding to a first principle: and so what

  is posterior in the order of becoming is prior in the order of nature.

  Now all things that go through the process of becoming acquire

  locomotion last. It is this that accounts for the fact that some

  living things, e.g. plants and many kinds of animals, owing to lack of

  the requisite organ, are entirely without motion, whereas others

  acquire it in the course of their being perfected. Therefore, if the

  degree in which things possess locomotion corresponds to the degree in

  which they have realized their natural development, then this motion

  must be prior to all others in respect of perfection of existence: and

  not only for this reason but also because a thing that is in motion

  loses its essential character less in the process of locomotion than

  in any other kind of motion: it is the only motion that does not

  involve a change of being in the sense in which there is a change in

  quality when a thing is altered and a change in quantity when a

  thing is increased or decreased. Above all it is plain that this

  motion, motion in respect of place, is what is in the strictest

  sense produced by that which moves itself; but it is the self-movent

  that we declare to be the first principle of things that are moved and

  impart motion and the primary source to which things that are in

  motion are to be referred.

  It is clear, then, from the foregoing arguments that locomotion is

  the primary motion. We have now to show which kind of locomotion is

  primary. The same process of reasoning will also make clear at the

  same time the truth of the assumption we have made both now and at a

  previous stage that it is possible that there should be a motion

  that is continuous and eternal. Now it is clear from the following

  considerations that no other than locomotion can be continuous.

  Every other motion and change is from an opposite to an opposite: thus

  for the processes of becoming and perishing the limits are the

  existent and the non-existent, for alteration the various pairs of

  contrary affections, and for increase and decrease either greatness

  and smallness or perfection and imperfection of magnitude: and changes

  to the respective contraries are contrary changes. Now a thing that is

  undergoing any particular kind of motion, but though previously

  existent has not always undergone it, must previously have been at

  rest so far as that motion is concerned. It is clear, then, that for

  the changing thing the contraries will be states of rest. And we

  have a similar result in the case of changes that are not motions: for

  becoming and perishing, whether regarded simply as such without

  qualification or as affecting something in particular, are

  opposites: therefore provided it is impossible for a thing to

  undergo opposite changes at the same time, the change will not be

  continuous, but a period of time will intervene between the opposite

  processes. The question whether these contradictory changes are

  contraries or not makes no difference, provided only it is

  impossible for them both to be present to the same thing at the same

  time: the point is of no importance to the argument. Nor does it

  matter if the thing need not rest in the contradictory state, or if

  there is no state of rest as a contrary to the process of change: it

  may be true that the non-existent is not at rest, and that perishing

  is a process to the non-existent. All that matters is the intervention

  of a time: it is this that prevents the change from being

  continuous: so, too, in our previous instances the important thing was

  not the relation of contrariety but the impossibility of the two

  processes being present to a thing at the same time. And there is no

  need to be disturbed by the fact that on this showing there may be

  more than one contrary to the same thing, that a particular motion

  will be contrary both to rest and to motion in the contrary direction.

  We have only to grasp the fact that a particular motion is in a

  sense the opposite both of a state of rest and of the contrary motion,

  in the same way as that which is of equal or standard measure is the

  opposite both of that which surpasses it and of that which it

  surpasses, and that it is impossible for the opposite motions or

  changes to be present to a thing at the same time. Furthermore, in the

  case of becoming and perishing it would seem to be an utterly absurd

  thing if as soon as anything has become it must necessarily perish and

  cannot continue to exist for any time: and, if this is true of

  becoming and perishing, we have fair grounds for inferring the same to

  be true of the other kinds of change, since it would be in the natural

  order of things that they should be uniform in this respect.

  8

  Let us now proceed to maintain that it is possible that there should

  be an infinite motion that is single and continuous, and that this

  motion is rotatory motion. The motion of everything that is in process

  of locomotion is either rotatory or rectilinear or a compound of the

  two: consequently, if one of the former two is not continuous, that

  which is composed of them both cannot be continuous either. Now it

  is plain that if the locomotion of a thing is rectilinear and finite

  it is not continuous locomotion: for the thing must turn back, and

  that which turns back in a straight line undergoes two contrary

  locomotions, since, so far as motion in respect of place is concerned,

  upward motion is the contrary of downward motion, forward motion of

  backward motion, and motion to the left of motion to the right,

  these being the pairs of contraries in the sphere of place. But we

  have already defined single and continuous motion to be motion of a

  single thing in a single period of time and operating within a

  sphere admitting of no further specific differentiation (for we have

  three things to consider, first that which is in motion, e.g. a man or

  a god, secondly the 'when' of the motion, that is to say, the time,

  and thirdly the sphere within which it operates, which may be either

  place or affection or essential form or magnitude): and contraries are

  specifically not one and the same but distinct: and within the

  sphere of place we have the above-mentioned distinctions. Moreover

  we have an indication that motion from A to B is the contrary of

 
motion from B to A in the fact that, if they occur at the same time,

  they arrest and stop each other. And the same is true in the case of a

  circle: the motion from A towards B is the contrary of the motion from

  A towards G: for even if they are continuous and there is no turning

  back they arrest each other, because contraries annihilate or obstruct

  one another. On the other hand lateral motion is not the contrary of

  upward motion. But what shows most clearly that rectilinear motion

  cannot be continuous is the fact that turning back necessarily implies

  coming to a stand, not only when it is a straight line that is

  traversed, but also in the case of locomotion in a circle (which is

  not the same thing as rotatory locomotion: for, when a thing merely

  traverses a circle, it may either proceed on its course without a

  break or turn back again when it has reached the same point from which

  it started). We may assure ourselves of the necessity of this coming

  to a stand not only on the strength of observation, but also on

  theoretical grounds. We may start as follows: we have three points,

  starting-point, middle-point, and finishing-point, of which the

  middle-point in virtue of the relations in which it stands severally

  to the other two is both a starting-point and a finishing-point, and

  though numerically one is theoretically two. We have further the

  distinction between the potential and the actual. So in the straight

  line in question any one of the points lying between the two

  extremes is potentially a middle-point: but it is not actually so

  unless that which is in motion divides the line by coming to a stand

  at that point and beginning its motion again: thus the middle-point

  becomes both a starting-point and a goal, the starting-point of the

  latter part and the finishing-point of the first part of the motion.

  This is the case e.g. when A in the course of its locomotion comes

  to a stand at B and starts again towards G: but when its motion is

  continuous A cannot either have come to be or have ceased to be at the

  point B: it can only have been there at the moment of passing, its

  passage not being contained within any period of time except the whole